When someone stubs their toe on a coffee table, sensory neurons in their toe are activated,transmitting information to the brain. The message goes through a neuron that runs from the toe up to the base of the spine. The neural signal continues through the spinal cord until it enters the brain and “Ow!---” the person recognizes that they stubbed their toe. The brain sends impulses back down to the muscles controlling the leg and foot, causing the poor soul to jump up and down - while holding their foot.
Parts of a Neuron
The nervous system is a network of neurons that bring sensory information to the brain and convey its instructions to the muscles. A neuron is a single nerve cell. Every neuron is made up of the same basic parts: dendrites, cell body (soma), axon, myelin sheath, synaptic knobs, and neurotransmitters. The dendrites are rootlike parts of the cell that stretch out from the cell body. They grow to make direct chemical and electrical connections with other neurons (synaptic connections). The cell body contains the nucleus and other parts of the cell necessary to sustain life. The axon is a cordlike structure that extends from the soma. The myelin sheath is a fatty covering around the axon that speeds neural impulses. Synaptic knobs, also called terminal buttons, are the branched end of the axon that contains neurotransmitters. Neurotransmitters are chemicals contained in terminal buttons that allow neurons to communicate. They fit into receptor sites on the dendrites of neurons, similar to how a key fits in a lock.
Neuron Diagram, Source: Organismal Biology
Other parts relevant include the synapse, oligodendrocyte, and the Nodes of Ranvier. The synapse is the space between the terminal buttons of one neuron and the dendrites of the next. The Nodes of Ranvier are gaps in the myelin sheath along the axon. The flow of ions through its sodium and potassium channels, regenerate the electric message. As a type of glial cell, oligodendrocytes, (pronounced uh-li-gow-den-druh-sites), are vital to helping neurons function. They form myelin sheaths around axons in the CNS*.
The Process of “Firing” a Neuron
Neuronal communication is an electrochemical process, so it is a process that consists of elements (chemicals) reacting to release electrons and generate electrical energy. In its resting state, a neuron has a slightly negative charge (-70mv) because mostly negative ions are within it and mostly positive ions surround the cell. The cell membrane of the soma is selectively permeable, meaning that it can differentiate between types of molecules and let some through while blocking others. This prevents the positive and negative ions from mixing.
Two neuron chain, Source: Khan Academy
The reaction starts when the synaptic knobs of the presynaptic neuron (neuron that starts the process, ‘neuron A’) are stimulated and begin to release neurotransmitters into the synapse (synaptic cleft depicted). The neurotransmitters fit into receptors sites, on the dendrites of the postsynaptic neuron (‘neuron B’). If the threshold is reached, so a satisfactory amount of neurotransmitters reaches the postsynaptic neuron, then its cell membrane becomes permeable and the positive ions rush into the cell. The change in charge (now +40mv) instantaneously spreads down the neuron and this electric message firing is called the action potential.
When the charge reaches the synaptic knobs of neuron B, they release their neurotransmitters into the synapse. The process continues if enough neurotransmitters reach the next neuron to pass the threshold. The all-or-none principle states that a neuron will either fire completely every time or not at all. It cannot fire a little or a lot, the impulse is the same every time.
Neuronal communication is how information gets to our brain and back. It is important in stimulating muscle contraction, creating an awareness of our environment, and plays a major role in emotion, memory, and reasoning. Nerve cells communicate by sending neurotransmitters that elicit an electrical impulse in a postsynaptic neuron and continue this chain until the brain. Since neurons can only send electrical impulses one way, from the soma through the axon, there are two pathways for information - allowing the brain to send directions back to the rest of the body. Nerve cell communication is an incredibly important process as it is vital to the brain and the basis of the nervous system.
Sources:
Barron’s AP Psychology 8th edition Textbook
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